ES2773667T3 - Coated panel, printed directly - Google Patents

Abstract

Panel, in particular wall, ceiling or floor panel, comprising a support layer (71) with a front side and a rear side, where the support layer (71), at least on its front side, starting from the front side above, it comprises the following layers: - a primer layer (72) obtained by application by means of a curtain coating process; - a decoration layer (73) comprising a polymerizable printing ink; and - a polymer layer (74) exhibiting a hardness gradient, such that the hardness of the polymer layer continuously decreases with increasing depth, observed from the polymer layer,

Description

DESCRIPTION

Coated panel, direct printed

1. Field of the invention

The present invention relates to a coated panel, in particular to a wall, ceiling or floor panel, as well as to a method for coating such a panel.

2. Background

A plurality of panels for wall, ceiling or wall cladding are known from the state of the art. In recent years, so-called laminate panels in particular have become widely used. Laminate panels are relatively cost-effective and can be worked well. In general, they are made on the basis of a support plate (support layer) made of MDF or HDF material, on whose surface a decorative paper impregnated with a resin is applied. A so-called "overlay" paper is also commonly placed on the decorative paper to improve the durability of the surface. The papers used are impregnated with resins, such as amino resins that harden by compression, under the effect of heat and pressure. To increase the abrasion resistance of the surfaces, the resins are also often provided with abrasion resistant particles.

In a further development of laminate panels of this kind, so-called direct-printed panels were developed. Papers are generally no longer used in those directly printed panels, in particular decorative papers are not used. Rather, the decorative layer is printed directly on the surface of the panel by using dispersion inks, by a hollow printing process; wherein the panel, for that purpose, is usually provided with a suitable primer layer. For this, the primer layer is usually applied by rollers. After the decorative layer has dried, several layers of resin are then applied and cured. The resin layers serve as a protective layer and as an abrasion resistant surface. To increase the abrasion resistance, abrasion resistant particles are also often incorporated here into the resin layer, usually corundum.

The application WO 2007/042258 A1 describes, for example, a process for the direct coating of a board made of wood material, in which, in a single coating step, a relatively thick protective layer of plastic material is applied to the surface of a plate. The plastic material used in this case is an acrylate system with polymerization capacity, which hardens through polymerization. The polymerization is activated by radiation, so that through the thickness of the applied layer a complete conversion takes place.

An improvement of the known state of the art is presented in the application WO 2008/061791 A1 of the same applicant. The central idea of the improvement of this document resides in the fact that, on the surface of a panel, two liquid layers of polymers are applied with the wet-on-wet technique, so that a partial mixing of the media takes place. coating. The two layers applied with the wet-on-wet technique are then hardened together, where the resulting coating, due to partial mixing, presents a hardness gradient, where the hardness of the coating decreases, with increasing depth, observed from the surface of the resulting coating,

US 2002/0081393 teaches to coat a wood or wood-like substrate with a series of layers, where at least one primer layer is applied to the substrate using a curtain coating. All applied coats must be cured before applying the next coat.

Document DE 102010011602 teaches the printing of a plastic film or a backing plate laminated with a plastic film with images or graphic patterns with an ink to be cured by UV radiation.

WO 2010/070474 discloses that a decoration layer can be applied directly or indirectly to a substrate, where it is possible for indirect printing to print the desired motif on a primer layer that is applied to the substrate. The printing ink can be a UV ink.

DE 10 2009 044 092 relates to a method for producing a digitally printed workpiece. Printed decorative paper will need to be drawn through a resin bath and then dried.

EP 2269744 refers to a panel made of wood-based material with a surface coating comprising a primer layer applied to the wood-based material and at least one lacquer layer cured by means of UV light.

Although in particular the application WO 2008/061791 represents a considerable advance in the state of the art, there is always a need for improvements. Therefore, the object of the present invention is to improve the previously known state of the art, and in particular to provide a coated panel, in which the adhesion of the layers is increased, and in this way to achieve the values particularly suitable in crosslinking test according to DIN ISO 2409.

These and other objects that are also mentioned in the following description or that can be observed by the person skilled in the art, are solved with a panel according to claim 1 and with a method according to claim 9.

3. Detailed description of the invention

A panel according to the invention comprises a support layer with a front side and a rear side. The support layer is for example a board made of a wood material, such as MDF (medium density fibreboard), or HDF (high density fibreboard), a chipboard, an OSB board (chipboard oriented) or the like. However, it can also be made of a plastic material, in particular PVC. At least on the front side of the support layer a layer system is provided which, viewed from the support layer outwards, comprises a primer layer, a decoration layer and a polymer layer, as a wear layer, which it preferably has a hardness gradient according to the application WO 2008/061791 described in the introduction, by the same applicant. The particularity of the decoration layer resides in the use of a polymerizable printing ink (ink), in particular based on a polymerizable acrylate. The decoration layer is preferably printed directly on the panel and thus consists practically of the printing ink. Alternatively, however, a printed paper can be used as a decoration layer (decorative paper), as is usual for laminate panels.

Dispersion inks, such as acrylate inks, are usually used in the state of the art as printing inks (ink). These printing inks are generally used in the hollow printing process, which is commonly used. Dispersion inks are printing inks that generally consist of three main components, namely (a) a solvent (usually water), (b) binders in the form of plastic polymers (acrylates), which are deposited in a together as the solvent evaporates and forms a solid layer, as well as (c) colored pigments to produce the desired coating and the desired color tint. Curing of these printing inks therefore does not take place through polymerization, but rather through volatilization of the solvent, since the binders are already present as polymers. During the evaporation of the binder, the polymers contained in the dispersion unite in a strictly physical way, forming a solid, closed layer.

Remarkably, applicants have now found that improved adhesion properties of the layer system can be achieved when conventional dispersion inks are not used, but polymerizable printing inks are used instead. The aforementioned applies in particular to the preferred application with a polymer layer, in particular with a hardness gradient. The positive effect is particularly marked when the printing ink of the decoration layer and the polymer layer are hardened or polymerized together (when the decoration layer is printed directly, as for example in digital printing, the decoration layer it is practically composed of the printing ink). As the curing of a polymer layer as well as a polymerizable printing ink (such as polymerizable acrylates, as well as UV-reactive inks in general), the chemical reaction that takes place during polymerization, in particular polycondensation, is understood here. A distinction must be made here between the drying of layers of this kind, in which only the solvent evaporates, such as the water content of the printing ink, or of the polymer layers, as well as it is strictly physically reduced. . Through the joint curing (polymerisation / polycondensation) of the acrylate, printing ink and polymer layer systems), a chemical crosslinking takes place at the boundary surface of the two layers, for which it is assumed to be responsible for the improved adhesion of the layers. The traditionally used dispersion printing inks do not have a polymerizable acrylate system, so that no such chemical crosslinking takes place between the printing ink, hence the decoration layer, and the polymer layer.

The polymerizable acrylates, as preferably used in the present invention, comprise as main components acrylate monomers, acrylate oligomers and optionally photoinitiators. The photoinitiators, under the effect of radiation, cause a polymerization of the monomers or of the oligomers, as a result of which the printing ink quickly hardens. The use of a printing ink based on a polymerizable acrylate, together with a layer of polymers applied on top (as a use layer), advantageously, it makes it possible to dispense with an adhesion activator layer, as was usually necessary in the state of the art, to improve the adhesion of the polymer layer on the decoration layer. It has been observed that the printing inks commonly used in the state of the art only achieve insufficient adhesion with the polymer layer applied on top, with a hardness gradient. In the state of the art, this disadvantage had to be overcome by using an adhesion promoter between the decoration layer and the polymer layer (wear layer).

The printing inks used for the present invention are polymerizable printing inks and in particular polymerizable acrylate systems. The polymerizable printing inks as main components contain binders, namely resins containing reactive double bonds; monomers or oligomers, such as acrylate monomers and acrylate oligomers; optionally photoinitiators for radiation-hardening printing inks; additives such as defoamers; flow promoter additives, thickeners, among others; color pigments such as phthalocyanine pigments, azo dyes, indigoid dyes, aniline black and / or carbon black; as well as loading agents to achieve certain physical technical properties. In general, preferably, the printing inks used for the present invention can be cured by radiation, as in particular the printing inks that can be cured by UV radiation (UV-curable). The printing ink is particularly preferably a polymerizable acrylate system that can be cured by UV radiation.

Preferably, the primer layer is based on an aqueous dispersion (preferably in an aqueous acrylate system), which has a suitable composition, so that it can be applied by a curtain coating process. In the state of the art, the primer layer, up to now, was usually applied on the surface of the carrier layer, in a roller application process. In such roll application procedures, the coating material (the primer layer) is discharged directly from the cylinder onto the surfaces to be coated or, indirectly, by means of a compression roll, is pressed onto the components. to be coated. The inventors of the present application have determined that the surfaces that were produced in this way, in particular in the case of precisely detailed print images, such as are commonly needed in the imitation of genuine wood surfaces, do not achieve the resolution and the sharpness you can really expect. In particular when using polymerizable printing inks, inconvenient streaks often occur in the printing image. The inventors, strikingly, have now determined that these problems can be avoided when instead of a roll coating process a curtain coating process is used. The rollers, in the roller application process, are supposed to be such that the coating material is applied directly from the roller, onto the surface to be coated, or indirectly, by means of a coating tape, they generate a ripple minimum surface area on the transferred coating due to its curvature. The ridges and valleys of this wavy surface, however, are so small that the surfaces thus produced can be printed well with traditional printing inks. However, it is assumed that the very low irregularity of the surfaces coated in this way, in the case of the use of polymerizable printing inks, is responsible for the problems mentioned. In any case, the primer layer can effectively prevent unwanted streaking in the case of the use of polymerizable printing inks (in particular in polymerizable acrylate systems).

The curtain coating processes and the corresponding installations are already known to the person skilled in the art from the state of the art (for example, from the application EP 1252937 A1), so that at this point a detailed description of the same. Important is the fact that in curtain coating processes a liquid curtain of coating material is produced, through which the components to be coated are conducted. Roller application does not take place.

The total thickness of the polymer layer (wear layer), after hardening, should preferably have a thickness of 20-300 pm, more preferably 40-250 pm, even more preferably 50-220 pm and most preferably 60-180 pm. Materials considered as preferred for the polymer layer are: 1,6-hexanediol diacrylate, polyester acrylate, polyurethane acrylate and dipropylene glycol diacrylate. Preferred values for the thickness of the primer layer (after curing) are 20-300 pm, more preferably 40-250 pm, even more preferably 50-220 pm and most preferably 40-250 pm. 60 - 180 pm. It is generally considered preferred that the primer layer preferably does not contain any lacquer that can be cured by ultraviolet rays. The thickness of the decorative layer is usually only a few nanometers to micrometers, preferably between 1 and 5 pm; more preferably between 2 and 4 pm, to achieve sufficient coverage of the ink.

In the manner mentioned in the introduction, according to the method according to the invention, the printing ink is cured together with the polymer layer applied on top (polymerized), preferably by means of a common radiation. Due to this, in the boundary layer between the printing ink and the polymer layer applied on top, a chemical crosslinking of the polymers used partially occurs. It has been observed that in this way it can particularly good adhesion of the polymer layer to the carrier layer is achieved. In the state of the art, until now it was always necessary to additionally use an adhesion promoter between the decoration layer and the applied polymer layer, to improve the adhesion of the polymer layer. With the preferred embodiment of the invention, not only can the adhesion promoter be dispensed with, but it has even been observed that the achieved adhesion of the polymer layer can be improved compared to conventional systems, based on adhesion promoters.

In the case of the crosslinking test according to DIN ISO 2409, the preferred embodiments of the invention achieve a characteristic crosslinking value of at least two, preferably at least one, and most preferably zero. These values naturally refer to the finished panel, when all layers are hardened.

The invention is explained below by means of some non-limiting embodiment examples:

Example 1: Panel with traditional indirect recess printing

In a first stage, an HDF support plate with a thickness of 8 mm, by means of a roller application mechanism, is provided with an adhesion promoter based on a commercially available aqueous acrylate dispersion. In the next stage, the plate is smoothed by means of a roller application mechanism, with a putty based on an aqueous dispersion of acrylate, with a high filler content. Subsequently, a printing base (primer layer) based on an aqueous dispersion of acrylate mixed with fillers and color pigments is applied, by a casting process (i.e. with a curtain coating process). After each of these coating stages, intermediate drying takes place at temperatures between 80 and 200 ° C. The plates treated in this way are led to a printing machine, essentially composed of an engraving roller and a rubber roller to transfer the printing image from the engraving cylinder to the plate. The printing image is generated by three printing apparatus connected downstream, whereby by means of each printing apparatus a self-dispersing printing ink, composed of color pigments and an aqueous acrylate dispersion, is applied. In the case of the imitation of dark walnut wood, for example 5 g / m2 of printing ink is applied. On the printing ink layer, in a subsequent process, a commercially available UV-adhesion promoter is applied by means of a roller application mechanism. Finally, the application of an oligomer that contains double bonds, provided with photoinitiators, takes place. On this layer, by means of a supporting sheet of the structure, a layer of another oligomer containing double bonds is placed, provided with photoinitiators, which can be hardened by radiation, and which is polymerized by means of ultraviolet radiation. After removal of the foil, the finished coated decorative plate is obtained, which can be separated into panels in a subsequent process, where known coupling elements can be added to these panels. The plate thus coated is subjected to a laboratory test. In the crosslinking test, a 3. In general, it applies here that the thicker the layer of printing ink must be applied to reproduce a decoration, the worse the result in the crosslinking test. This makes it difficult to mill the profiles for the coupling elements, because the polymer layer quickly breaks at the edges.

Example 2: Panels in which the printing image is produced with printing inks that are cured by radiation

Again an HDF support plate with a thickness of 8 mm is used and, as described in Example 1, it is provided with an aqueous adhesion promoter, putty and a printing base. On the plate thus treated, by digital printers, the same decorative image is produced as in Example 1. In this case, however, no dispersion inks are used, but digital printing inks that are cured by ultraviolet rays. An amount of ink of approximately 2 g / m2 is required to produce the print image. The ink is first fixed with 150 mJ / cm2 (mercury). This is followed by the application of 2 g / m2 of a first layer that is cured by ultraviolet rays, which mainly contains dipropylene glycol diacrylate. On this non-irradiated layer, an oligomer containing double bonds is applied, mixed with photoinitiators, as in example 1. Then, on that layer, by means of a sheet that forms a structure, a second layer of oligomers is placed, as in the Example 1. The set of layers is brought to a source of UV radiation and in this way the radiation-hardening layers are polymerized. The resulting polymer layer comprises the printing ink and all layers on top of it. Panels produced from this base are further subjected to a laboratory test. In the crosslinking test, no chipping of any kind occurs within the different planes of the coating. At most, removing the adhesive tape can damage the HDF support plate itself. The present invention also relates to a process for coating a panel. In this process according to the invention, a primer layer is applied to a carrier plate by means of a curtain coating process. After an optional drying of the primer layer, a decoration layer is then applied using a polymerizable printing ink, for example based on an acrylate polymerizable, preferably in a digital impression, that is, it is printed directly. As decoration, all the usual decorations are possible, such as in particular the imitation of real wood surfaces. Then, in an optional step, a partial curing of the printing ink can take place. Thus, at least 10% of the polymerizable acrylate in the printing ink has to be cured, but less than 50%, more preferably less than 30%, even more preferably less than 20%. Such partial curing has been found to lead to improved results in the crosslinking test. After the application of the decoration layer, a first liquid coating medium (first polymer) is applied on the not fully cured printing ink, and in a second stage at least one second liquid coating medium (second polymer) is applied. it is applied to the first coating medium that is still wet, so that partial mixing of the coating medium takes place at the boundary surface. In a subsequent step, the coating means, as well as the two applied coating means, are cured together with the printing ink by radiation. As a hardness gradient is desired, the first and second coating means should not be completely mixed, where after curing the hardness of the coating decreases with increasing depth, as observed from the surface of the resulting coating. This is explained in more detail in the description of the figures.

The data provided in relation to the description of the panel naturally applies analogously also for the method according to the invention. The aforementioned applies in particular to the data relating to the preferred materials for the primer layer, the printing ink and the polymer layer, their thicknesses, etc.

In the aforementioned manner, a central idea of the invention resides in the joint curing of the coating means (polymer layer) and the polymerizable printing ink, in particular a printing ink comprising a polymerizable acrylate. In other words, the coating medium is applied in liquid form onto the not yet cured printing ink, so that here too, in the boundary area, a certain penetration of the various materials takes place. In the subsequent co-curing, in this way, chemical bonds are formed between the printing ink and the coating means, which, according to the inventors, leads to the particularly desirable mechanical strength values of the panels coated with that way. In the state of the art, adhesion took place essentially only by physical processes, since there is no co-hardening, ie polymerization, between the printing ink and the applied polymer layers.

For the coating means of the first, as well as the second and optionally more polymer layers, a single polymerizable substance or mixtures of substances of this kind can be selected. Particularly suitable substances include acrylates capable of polymerisation in general, and in particular substances: 1,6-hexanediol diacrylate, polyester acrylate, polyurethane acrylate and dipropylene glycol diacrylate. A mixture of 1,6-hexanediol diacrylate and polyester acrylate is particularly suitable for the first layer, and for the second layer a mixture of polyurethane acrylate and dipropylene glycol diacrylate. In addition, additives, such as fluidizing aids, wetting aids, colorants, abrasion resistant particles, etc., may also be present in the coating media. It is considered essential that these other components allow the partial penetration described above and, thus, the crosslinking of the first and second layers; and further polymerization is possible if a hardness gradient is desired.

Generally preferably, the primer layer itself contains colorants, so that the applied primer layer has a suitable homogeneous color. Preferably, that color is selected in accordance with the decoration that will be applied later. For example, if the decoration layer is to represent a dark oak wood, then the primer layer is preferably provided in a corresponding dark brown shade.

4. Detailed Description of Illustrative Embodiments

A detailed description of illustrative embodiments is provided below by means of the accompanying diagrams and figures. Thus, FIGS. 1 to 6 explain the production of a polymer layer with a hardness gradient, analogously to application WO 2008/061791.

Figure 1 is a schematic representation of a coating process for producing a hardness gradient polymer layer according to the state of the art;

Figures 2a to 2c are schematic representations in which the development of the mixing of two liquid layers is represented;

Figures 3 to 5 are diagrams in which the hardness profile is represented as a function of the depth of the coating;

Figure 6 shows an illustrative panel, according to the invention, in a schematic assembly view.

In figure 1, schematically, a cladding installation for cladding wood material boards 10 is shown. Wood material boards 10, such as solid wood boards, HDF boards, MDF boards or chipboard boards, are conducted by means of a roller conveyor system 12, through the different stations of the coating installation. In a first coating station 14, by means of a rotating application cylinder 15, a first liquid coating medium 20, in a continuous coating, is applied on the plates of wood material 10. The application cylinder 15, by means of a device of supply 16, is supplied with coating medium. In the second coating station 17, by means of another rotating application cylinder 18, a second coating medium 21 is applied to the first coating medium 20, still wet. The application cylinder 18, by means of a supply device 19, is supplied with the second liquid coating medium. Of course, the application can also take place by any other suitable application method, such as by means of a spraying device, a coating knife or the like. It is only important that the application of the second layer takes place as long as the first layer is still sufficiently wet, so that a partial mixing of the layers takes place. Furthermore, after the second coating station 17 of course other coating stations may be provided, for example for applying a third liquid coating medium on the still liquid second coating medium 21 or also additional stations for introducing abrasion resistant particles on or in wet layers. After leaving the coating station 17, the coated plates 10 are transported to a curing station 30, where the layers are cured by UV irradiators 31. On the way from the coating station 17 to the curing station 30 a partial mixing of the liquid coating means 20 and 21, which in particular takes place at the boundary surfaces of the two coating means. Naturally, the mixing is the greater, the closer it is to that boundary surface of the two layers. By curing the layers in the curing station 30, the mixing process is stopped and the previously regulated mixing ratio and thus the mechanical properties of the coating produced are determined. The amount of mixing at the boundary surfaces - which takes place spontaneously and preferably without mechanical action from the outside - depends on the period that elapses between the application of the second coating medium 21 on the first coating medium 20 which is still wet and hardening at the curing station 30. Furthermore, the mixing of the two coating media is influenced by the respective viscosity of the coating media, where as a general rule it applies that the higher the viscosity, the lower the mixing by unit of time.

The principle of mixing the two applied coating means can best be seen on the basis of the schematic representation of Figures 2A to 2C. Figure 2A shows the state of the two coating means 20 and 21 applied to a wooden material plate 10, immediately after the application of the second coating means 21. At that time practically no mixing has yet taken place. Coating means 20 and 21, in this case, are polymers that respectively have different amounts of C-C carbon double bonds. As schematically indicated in Figure 2A, the first coating medium 20 has fewer CC double bonds than the second coating medium 21. Due to the higher number of CC double bonds in the coating medium 21, after the curing it will have a higher hardness than coating medium 20, which is provided with fewer CC double bonds.

Since the two coating means 20 and 21 are applied with the wet-on-wet technique, starting from the boundary surface 22, a mixing of the same occurs between the two layers, as indicated in FIG. 2B. This means that more double bonds are present in the area close to the boundary, relative to the boundary layer 22, by the mixing process, more double bonds are present in the layer below, and correspondingly in the area close to the boundary, the top layer slightly less double bonds, as before mixing. Figure 2C shows the two layers after mixing has progressed a little further and has reached a suitable degree of mixing. If curing of the coating means takes place at that time, for example by UV radiation, this degree of mixing is determined, since no mixing can take place any more in naturally hardened layers.

In the diagram of Figure 3 the hardness profile of a hardness gradient coating is marked. The hardness gradient example consisted of a plate of sanded wood material provided with a primer layer, on which the two different coating media were applied, using the wet-on-wet technique. The first coating medium applied consisted of approximately 35% 1,6-hexanediol diacrylate, and approximately 65% polyester acrylate, and was applied at 45 g / m2. The second coating medium that was applied over the first layer, still wet, was composed of approximately 70% polyurethane acrylate and approximately 30% dipropylene glycol diacrylate, and was applied at 40 g / m2. After the application of the second coat, 10 seconds were waited to allow the viscous liquid materials to mix. The two layers were then fully cured together.

In the example without a hardness gradient, several thin layers of material were applied individually and the previously applied layer hardened between respective application processes. The lower 3 layers were made up of a mixture of 70% polyester acrylate and 30% 1,6-hexanediol diacrylate, each with an application thickness of 12 g / m2 The top two layers were made up of 70% polyurethane diacrylate - glycol and 30% dipropylene acrylate, and the two upper layers contained 15% corundum, with a mean particle size of D 50 of 25 pm.

The test was carried out according to the European standard for laminate flooring DIN EN 13329, with a Taber Abraser 5151 tester from Taber Industries. Respectively after 200 rotations with S-41 sandpaper the hardness and depth of the marks of the samples were determined. The determination of the Martens hardness (hardness test which is recorded under the effect of the test force) was carried out in accordance with DIN EN ISO 14577. A "Fischerscope H100" from Helmut Fischer GmbH was used as the test apparatus. The following test parameters were used: Maximum force: 50/30 mN, as well as duration of the measurement: 20 seconds. The determination of the depth of the marks was carried out with a mechanical measuring profilgraph. A Perthometer S3P roughness tester from Perthen was used as the test apparatus.

In the measurement of the samples it was observed that probably due to the relatively soft materials used more or less deviations in the hardness of a given depth of the layer occurred. Therefore, it is necessary to measure at several points, in order to obtain representative data, which allow making statements, by forming an average value. In the measurements carried out, the hardness was measured, as well as the depth of the marks after 200 rotations of the sandpaper, respectively at four points. Four measurement points have been found to offer sufficient precision in most cases. Naturally, even more accurate measurement results are obtained when more than four measurement points are used, for example eight.

The table below shows the individual measured values for the example sample according to the invention. The measurement took place on the fully cured coating, that is, in the state in which corresponding products would also be used as floor panels in reality.

Table 1: Examples with hardness gradient

In the table presented above, the "Rotation" column indicates the number of rotations that were performed with the Taber Abraser test apparatus. The "Depth of Marks" column indicates how many microns of coating material, starting from the original surface, was worn away at the four measurement points 1-4. The column "Hardness measurement - depth" indicates by how many micrometers the test mandrel has penetrated the coating respectively at the four measurement points 1-4. In the column "Martens hardness" the hardness in newtons per mm2 is respectively indicated for the four measuring points 1-4. Below the individual values, the respective mean value for the four measuring points is indicated. From the table presented above it can be clearly seen that the Martens hardness decreases the deeper the finished, hardened layer is penetrated. It is also observed that in the case of 800 and 1000 rotations (total) it can be registered a slight increase in Martens hardness. This can be attributed to uneven mixing of the two coating means used, which in practice can only be completely avoided in a complicated way.

However, it can be clearly seen from the diagram of FIG. 3 that in the overall hardness gradient example an almost continuous reduction in hardness is present, without large discontinuities. The comparative example without a hardness gradient, on the other hand, does not show a continuous hardness profile, but instead exhibits a marked crack point down to the original initial hardness at a depth of 60 to 80 pm.

The average values of the test body of the invention are shown in the following table 2.

Table 2: Average values of the example with hardness gradients

The values of the comparison test body without hardness gradient are represented in tables 3 and 4 below.

Table 3: Sample without hardness gradient

Table 4: Mean values of the sample without hardness gradient

Experimentally it has been found that particularly desirable mechanical properties of the finished total layer can be achieved when the hardness gradient of the finished total layer - as shown by way of example in Figure 3 - corresponds essentially to the following relationship:

(-3.0 • jc) C <7 (x) <(-0.2 • x) C

where:

x is the absolute value of the depth in pm of the coating, observed from the surface of the coating;

Y ( x) is the absolute value of the hardness in N / mm2 in the case of a given depth x; and

C is the absolute value of the initial hardness in N / mm2 of the coating in the case of approximately x = 0-5 pm deep.

As the "absolute" values, it is understood that in the above formula only strictly numerical values are entered, therefore, without the corresponding unit of measure "pm" or "N / mm2". For example, if the initial value of the previous example with hardness gradient amounts to 140.8 N / mm2 (see table 2), then only the absolute values are included in the table above, therefore C = 140.8. Similarly, for x only absolute values are included, so for example x = 3.5. This results in, for example, upper and lower limits for Y (x = 3.5) of 140.1 or 130.3. In the case of a depth of x = 40 pm, then, for example, 132.8 is obtained for the upper limit as well as 20.8 for the lower limit. Those upper and lower limits for Y (x) have the unit of measure N / mm2. It is important that the absolute values are used in the formula starting from the units of measurement indicated "pm" or "N / mm2" and not for example starting from "mm" or "N / m2". It should be clear to the person skilled in the art that the above formula is not a mathematical formula to describe the hardness gradient itself, but rather defines a range in which it should extend.

The initial value of the hardness of the coating is the value in the first few pm of the coating. Due to commonly used measurement methods, using a test mandrel penetrating a few pm into the coating, it is difficult to determine the hardness for the penetration depth "0 pm". Therefore, the formulation "essentially" is selected because it is difficult to achieve a perfectly uniform mixing of the materials, so that in reality small anomalies can often occur, such as the hardness value of 104.2 newton / mm2 in the case of a depth of 42.1 pm (see table 2) of the above example with hardness gradient. Furthermore, the values extremely close to the surface of the wood material plate are generally inaccurate, since the thickness of the residual layer to be measured must have a certain minimum thickness to enable convenient measurements. The thickness of the residual layer, for convenient measurements, should therefore be at least 5 pm, preferably 10 pm and even more preferably 20 pm. In other words, the last 20 pm of the layer, near the wood material plate, should not necessarily follow the preferred hardness gradient indicated above, although this is naturally considered preferred.

In a preferred embodiment, the hardness gradient essentially follows the following relationship:

(-2.5 • x) C <7 (x) <(-0.4 • x) C

And in an even more preferred embodiment, essentially:

(-2, 0 • x) + C <7 ( ^jc ) <(-0, 6 • x) C

In Figures 4 to 6, the meaning of the relationships of the hardness gradients, indicated above, is explained by the example with a hardness gradient. It should be clear that the stated absolute values for hardness and depth are for illustrative purposes only. Naturally, it is also possible to apply full coats with markedly greater thicknesses or with smaller thicknesses. Furthermore, the absolute value of the hardness naturally depends on the materials used and may optionally be higher or lower than the values of the hardness gradient example. However, the order of magnitude of the values reported for the hardness gradient example is considered especially preferred and suitable for use on a floor panel.

Figure 6, in a schematic representation, shows a panel according to the invention. The panel shown is a floor panel and comprises a support layer 71 of MDF. Figure 7 is not made to scale: the support layer 71 in practice has a thickness of several millimeters, while in particular the layers provided on the front side in total have a thickness of only a few hundred micrometers in total. On the front side of the carrier layer 71 a primer layer 72 is applied. The primer layer 72 is based on an aqueous acrylate system and is applied with a curtain coating process. After the primer layer 72 has dried, by means of digital printing, using a polymerizable printing ink, the decoration layer 73 is applied. Optionally, between the primer layer 72 and the decoration layer 73 other layers, such as in particular a suitable adhesion promoter layer. Similarly, optionally, between the carrier layer 71 and the primer layer 72, other layers, such as in particular putty layers, but also other adhesion promoter layers, may be provided to improve an adhesion of the primer layer. 72 on the front side of the support layer 71. Those other layers of adhesion promoter and putty are known to the person skilled in the art, so a detailed explanation of them is omitted here. Directly on the decoration layer 73, for which a polymerizable printing ink was used, for example based on a polymerizable acrylate, a layer of polymers 74 is applied which, as explained in the introduction, exhibits a hardness gradient. . The decoration layer 73, as well as the polymer layer 74, are jointly cured. For this, the layers preferably have photoinitiators, so that, for example, by UV irradiation, polymerization and thus curing of the two layers 73 and 74 take place. Optionally, on the polymer layer 74 hardened, another thin layer 75 of a gloss lacquer is applied.

On the back side of the support layer 71, a thin layer of impact noise insulation 76 is provided. The impact noise insulation may be a 1 to 2 mm thin non-woven fiber fabric. As a finishing layer, a foil 77 is further provided on the rear side of the carrier layer, which serves as a protection against moisture.

Claims (15)

1. Panel, in particular wall, ceiling or floor panel, comprising a support layer (71) with a front side and a rear side, where the support layer (71), at least on its front side, starting from the anterior side comprises the following layers: - a primer layer (72) obtained by application by means of a curtain coating process; - a decoration layer (73) comprising a polymerizable printing ink; and - a polymer layer (74) exhibiting a hardness gradient, such that the hardness of the polymer layer continuously decreases with increasing depth, observed from the polymer layer, Panel according to claim 1, characterized in that the printing ink is based on a polymerizable acrylate. Panel according to any one of the preceding claims, characterized in that the printing ink of the decoration layer (73) and the polymer layer (74) have been cured together, preferably by radiation. Panel according to any of the preceding claims, characterized in that the hardness gradient corresponds to the following relationship: (-3, 0 • x) C <Y ( x ) <(-0, 2 • x) + C where: x is the absolute value of the depth in pm of the coating, observed from the surface of the coating; Y ( x) is the absolute value of the hardness in N / mm (i) 2 in the case of a given depth x; and C is the absolute value of the initial hardness in N / mm2 of the coating in the case of approximately x = 0-5 pm deep. Panel according to claim 4, characterized in that the hardness gradient basically corresponds to the following relationship: (-2.5-x) C < Y ( x) < (-0.4-x) C; and preferably (-twenty • x) + C < Y {x) <(-0, 6 • x) + C. Panel according to any of the preceding claims, characterized in that the polymer layer (74) has a thickness of 20 to 300 pm, preferably 40 to 250 pm, more preferably 50 to 220 pm and most preferably of 60 to 180 pm. Panel according to any of the preceding claims, characterized in that the primer layer (72) has a thickness of 20 to 300 µm, preferably 40 to 250 µm, more preferably 50 to 220 µm and most preferably of 60 to 180 pm. Panel according to any of the preceding claims, characterized in that the primer layer (72) is based on an aqueous acrylate system. 9. Procedure for coating a panel, comprising the following stages: (i) provision of a support plate; (ii) applying a primer coat by a curtain coating process; (iii) application of a decoration layer by means of a polymerizable printing ink, in particular based on a polymerizable acrylate; (iv) optionally: partial curing of the printing ink; (v) applying a first liquid coating medium over the not fully cured printing ink; (vi) application of at least one second liquid coating medium on the still wet first coating medium, so that partial mixing of the coating means takes place; (vii) hardening of at least the applied coating medium and the printing ink together by radiation so that the cured coating media exhibits a hardness gradient, where the hardness of the coating decreases with increasing depth, seen from the surface of the resulting coating. 10. Method according to any of the preceding method claims, characterized in that the coating means are applied directly to the decorative layer. 11. Process according to any of the preceding claims, characterized in that step (vii) is carried out so that the hardened coating means present a hardness gradient, where the hardness of the coating decreases with increasing depth, observed from the surface of the resulting coating, and where the hardness gradient corresponds to the following relationship: (-3, 0 • x) + C <Y {x) <(-0, 2 -x) C where: x is the absolute value of the depth in pm of the coating, observed from the surface of the coating; Y ( x) is the absolute value of the hardness in N / mm2 in the case of a given depth x; and C is the absolute value of the initial hardness in N / mm2 of the coating in the case of approximately x = 0-5 pm deep. 12. Method according to the preceding claims, characterized in that the hardness gradient corresponds to the following relationship: (-2, 5 • x) C <Y ( x) < (-0, 4 -x) C; and preferably (-2, 0 • x) C <y (x) <(-0, 6 -x) C. Method according to any of the preceding claims, characterized in that the primer layer is applied in a thickness of 20 to 300 pm, preferably 40 to 250 pm, more preferably 50 to 220 pm and most preferably 60 at 180 pm. Method according to any of the preceding claims, characterized in that the primer layer is not a UV-curable lacquer. Panel or method according to any of the preceding claims, characterized in that the decoration layer (73) consists of the printing ink.